Optical fibers are commonly used in sensors. Typically, some form of energy that is desired to be sensed is received by the optical fiber. The energy causes changes in the optical fiber disposition, location, configuration that manifest themselves, for example, as microbends. Light being transmitted through the optical fiber experiences changes in amplitude or phase due to these changes.
Strain sensors have been developed to sense localized strain, that is, the strain present at discrete locations along the optical fiber. This localized strain corresponds to the energy present at a particular location along the fiber. One such strain sensor is described in U.S. Pat. No. 4,459,477 to Asawa et al. (hereinafter Asawa et al.) where optical pulses are transmitted into the optical fiber. Microbend transducers dispersed along the exterior of the optical fiber cause changes in amplitude of the transmitted optical pulses. The change in amplitude experienced by the optical pulses corresponds to the energy present with respect to a microbend transducer in a localized portion of the optical fiber. However, significant problems with the use of microbend transducers in optical fibers are the higher power losses, transient effects and complex apparatus required to measure the strain.
Another patent that relies on discontinuities to measure strain is U.S. Pat. No. 4,653,916 to Henning et al. (hereinafter Henning et al.). Henning et al. and Asawa et al. rely on discontinuities which are produced by external means such as transducers 14 in Asawa et al. This is a relatively complex, burdensome and obtrusive system requiring extensive external apparatus to measure localized strain. Because the discontinuities are largely external, strain at various locations throughout the cross-section of a structure cannot be measured. In the invention disclosed herein, the system of optical fibers and splices can be located throughout the cross-section of a structure to yield an array of strain measurements. This is accomplished without the type of power loss constraints that other sensors have heretofore experienced.
The strain sensor of the invention is comprised of an optical waveguide utilized with a source for providing optical pulses to the waveguide, and a system for sensing localized strain in the optical waveguide. The sensing system measures changes in arrival times of the optical pulses, which shifts correspond to the localized strain. In a preferred embodiment the sensing means includes reflecting a portion of the optical pulses at predetermined positions along the optical waveguide and detecting the shift in arrival times of the reflected optical pulses.
In utilizing the splices and waveguides of the invention, portions of the sensing system embedded can actually be added in the structure to be measured, often without adversely affecting its structural integrity. Furthermore, the efficiency with which the system operates renders it more economical to the user and more amenable for use with various structures.
The above is a brief description of the deficiencies of the prior art and advantages of the invention. Other advantages will be apparent from the Detailed Description of the Preferred Embodiment which follows.